Spark plug



Jul 21, 1936. RABEZZANA 2,048,029

SPARK PLUG Filed Feb. 6, 1935 22 02 01 Wafiezzazza Patented July 21, 1936 UNITED STATES SPARK PLUG Hector Rabezzana, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application February 6, 1935, Serial No. 5,188 3 Claims. (01. 123-169) My invention relates to spark plugs for use with internal combustion engines, and particularly to spark plugs designed for use with internal combustion engines wherein the pressure 5 to which the combustible mixture within the engine cylinder is compressed to an unusually high pressure prior to the ignition thereof. Such engines are commonly called high compression engines, they are used particularly in airplane service, and the insulating means for the central electrode of the plug is ordinarily made of mica.

In investigating spark plugs used with engines wherein the gas pressure just before ignition is inordinately high I have observed that there is often a failure to ignite the combustible mixture,

at the starting ofv the engine which failure to ignite, however, ceases after a short time, and the plug thereafter operates properly and without misfiring of the charge. This improper action at starting indicates leakage of current from the central insulated electrode of the plug to the grounded shell thereof whereby the plug is short circuited to a greater or less degree, with consequent reduction in the igniting efiect of such sparking as may occur at the spark gap; which leakage as my investigations proveis caused by gas inside the central insulated electrode assembly and between the electrode proper and metallic supporting means therefor such as is ordinarily present in spark plugs wherein mica insulation is used; which supporting means in such'plugs is in direct metal to metal contact with the grounded shell of the plug. Such gas may be present in spaces not completely filled by the mica 5 insulating material, or within the insulation it self which is built up of laminae or sheets of mica, and in either case may be properly referred to as gas within the interstices of the grounded central electrode assembly; and said gas is at substantially atmospheric pressure at starting, and after the engine has been out of operation for a short time.

Misfiring while present at starting ceases after a time, this being due, as I have found by ex- 45 tensive experimentation, to leakage of gas from the high pressure charge inside the engine cylinder into the interstices aforesaid, thus equalizing the pressure between them and the pressure inside the cylinder; and the object of my invention is to provide for rapid equalization of thepressure inside the cylinder and within the unavoidable space, that is the interstices as a whole, within the central electrode assembly (meaning the central electrode itself, the insulation there- 55 for, and the grounded metallic supporting means for the electrode and insulating material) so that misfiring will not occur at the starting of the engine. This rapid equalizing of pressures is brought about by a passage extending through an element or part of the grounded metallic 5 supporting means for the central electrode whereby pressure within the engine cylinder is communicated to the gas within the interstices aforesaid inside the central insulated electrode assembly of the plug.

The drawing accompanying and forming a part of this application illustrates the preferred form of my invention, and the claims at the end of this specification enumerate the particular features wherein the same consists. My invention, 15 however, may be embodied in various forms other than the particular form illustrated and hereinafter described, so long as such other forms come within the scope of the concluding claims.

In the drawing: 20

Figure 1 is an enlarged view showing a section upon a central plane extending longitudinally thereof, of a spark plug made in accordance with my invention.

Figure 2 isa view further enlarged, showing 25 a section upon a transverse plane indicated by the line 2-4, Figure 1.

Figure 3 is a perspective view showing a metallic bushing, which forms a part of the grounded metallic supporting means for the insulating 30 member and central electrode of the plug.

Figure 4 is a perspective view showing the lower end of the central insulated electrode of the plug.

Referring now to the drawing, the numeral 6 designates the metallic shell or casing of my improved spark plug which is adapted to screw into a threaded opening inan internal combustion engine wall, and I and 8 the grounded shell electrodes of the plug. I 40 The central electrode assembly of the plug comprises a single unitary structure or device separate from the shell or casing, and in the plug illustrated the same is readily removable from the shell and replaceable therein, as a unit; although said assembly may be permanently assembled with the shell as in so-called single piece spark plugs. In the-plug illustrated the central insulated electrode 9 is supported from an annular metallic member in the form of a gland nut III which screws into the threaded upper part of the inside of the shell 6; a metallic packing gasket ll being ordinarily placed between'the lower end of said gland nut and an internal shoulder of the shell. The gland nut is in direct metal to metal contact with the shell and is therefore grounded as will be understood.

The central electrode has a terminal connector I2 at its upper end and a head l3 at its lower end arranged in sparking relation to the shell electrodes 1, 8 as shown; and the same extends through the gland nut ill, but is insulated therefrom by insulating means shown as made up of several insulating members made, preferably, of mica. That is, in the plug shown the central electrode is surrounded by a tubular insulating member I4 made of sheet mica and which extends throughout substantially the whole length thereof; and insulating members l5, l6 made up of stacks of mica washers surround the ends of the tubular insulator and are located between the upper and lower ends of the gland nut I 0 and the terminal connector 12 and head i3 of the central electrode 9. A metallic bushing I1 is preferably interposed between the upper end of the lower insulating member l6 and the lower end of a metallic sleeve I8 which surroundsthe tubular insulating member H and fits within a seat provided for it within the gland nut; but inasmuch as the bushing, the sleeve and gland nut are all of metal and contact with one another the three provide a grounded support for the insulating members and for the central insulated electrode of the plug or, as otherwise stated, provide grounded supporting means for said electrode.

The parts which form the central electrode assembly are preferably assembled under pressure applied to the terminal connector l2 and the head i3 and acting longitudinally of the electrode, and they are held in assembled relation by collapsing the connector against the upper end of the electrode while said pressure is maintained, all in substantial accordance with United States patent for Method of forming mica insulated spark plugs, No. 1,872,497, August 16, 1932; during which assembling the connector is heated by a current of electricity the volume of which, preferably, is sufficient to accomplish the welding thereof to said electrode. The pressure equalizing feature of my invention is, however, in no way dependent upon the particular way in which the parts of the central electrode assembly are assembled and held together to form a single unitary structure.

While in a central electrode assembly made up of the parts enumerated the total volume of unavoidable uniilled space, or interstices, between the electrode itself and the grounded metallic supporting means therefor, and which interstices are. variously and indefinitely distributed within said assembly, is very small, there is nevertheless an appreciable space within the assembly which is initially or becomes filled with gas, and which gas offers much less resistance to leakage of current than would be the case if no such gas-filled space was present. For example, it is impossible to secure perfect contact between the lower insulator l6 and the lower end of the tubular insulator l4, and there is an appreciable gas containing space, as at I9, between said parts which forms a path of less resistance to leakage of high tension current from the head l3 to the bushing l'l, sleeve I8, gland nut Ill and to the casing 6 of the plug, than would be the case if said space H was not present; and

- |5and the upper end of said tubular insulator,

leakage along this space being from the terminal connector l2 to the sleeve l8, gland nut l0 and likewise to the casing 6 of the plug.

The sum total of the interstices within the confines of the central electrode assembly there for contain an appreciable quantity of gas which provides a path of much less resistance to the flow of high tension current than mica, or any of the insulating materials usually employed in spark plugs, would provide, were the interstices completely filled with insulating material. If the plug sparks properly at atmospheric pressure, or if it is used in an engine employing only moderate compression, the resistance of the path provided by the gas within the interstices, together with its considerable length, will ordinarily prevent leakage to an extent suflicient to reduce the current at the spark gap below that required for ignition of the combustible mixture. When, however, such plugs are used in high compression engines the gas pressure at the spark gap is often sufficient to prevent the high tension current from jumping the gap (this because the resistance of a gas increases as its pressure increases) and short circuiting through the gas-filled interstices, or excessive leakage such as to greatly reduce the current at the gap, will occur; the result in either case being misfiring of the plug and a .failure to ignite the combustible mixture within the engine cylinder.

My invention therefore aims to secure a gas pressure within the interstices aforesaid suillciently high to insure that the flow of current shall be (disregarding unavoidable leakage) across the spark gap, rather than through the gas within the interstices of the central electrode assembly; and to secure that result as quickly as possible upon the starting of the engine. Leakage through the lower insulating member I 6 and along the joints between the ends thereof and the head l3 and bushing I1 will sometimes result in the building up of pres sure within the interstices, but this will be a slow process; and an object of my invention is to build up such pressure instantly, or within an extremely short time. Furthermore, the bet ter the plug is made the slower will be the leakage here referred to; so that my invention is of particular importance in high grade plugs designed for use with airplane engines.

The building up of pressure within the interstices of the central electrode assembly to a pressure corresponding or corresponding approximately with the pressure within the cylinder of the engine is secured by providing the grounded metallic supporting means for the central electrode, which formsla part of said assembly, with a passage, which I call an equalizing passage". the outer end of which is in open communication with the interior or the engine cylinder when the plug is in use, and the inner end of which communicates with said interstices. Thus in the plug herein disclosed the gland nut III, the sleeve IO and the bushing I! together with the tubular insulating member II and the insulators I! and I8 collectively form grounded metallic supporting means for the central electrode; and one or more very small holes 2|, or grooves '22, or both, are provided in the bushing [1, whereby a passage or passages are provided leading from the interior of the engine cylinder into the interstices inside the central electrode assembly. The interstices within the central electrode assembly communicate with one another and form in effect one single space or cavity, so that pressure reaching any part of the gas containing chamber thus provided is equalized throughout every part thereof whatever may be the location and disposition of said parts.

There is little or no actual flow of gas through the passage 2| or 22 as the total volume of interstices is minute, and the cross-sectional area of said passage may be as small as it may be conveniently made; so long as the area and arrangement thereof is such that it will not become clogged by oil or carbon deposit when the plug is in use.

However the equalizing passage is formed and whatever may be its location in the grounded supporting part of the central electrode assembly it is obvious that pressure inside the engine cylinder is communicated to the gases within the interstices aforesaid, thus building up gas pressure therein which approaches the maximum pressure within the engine cylinder; and which will always be suillciently high to prevent short circuiting of the plug or leakage of current to an extent suillcient to interfere with the proper operation thereof. The actual flow of gas through the equalizing passage will be slight as hereinbefore explained but, and because of the presence of an actual passage even though its area may be small, the equalizing action will take place without appreciable delay as the gas pressure within the engine cylinder approaches its maxi mum, so that there will be no failure of the plug to ignite the combustible mixture even though at starting the pressure within the interstices will ordinarily be atmospheric pressure.

Having thus described and explained my invention I claim and desire to secure by Letters Patent:

1. In a spark plug, a central electrode having a terminal connector at its outer end, and the inner end of which terminates adjacent the shell electrode of the plug; annular metallic means through which said central electrode extends, and which annular metallic means includes a tubular bushing at its lower end, said metallic means being in direct metal to metal contact with the metallic shell or casing of the plug; and insulating means whereby said central electrode is insulated from said metallic means and which metallic and insulating means together serve to support said central electrode; said tubular bushing having an equalizing passage whereby pressure inside an engine cylinder, and pressure within interstices between said central electrode and said supporting means therefor, are equalized when the plug is in use.

2. In a spark plug, a central electrode having a terminal connector at its outer end and an enlarged head at its inner end; a tubular insulating member surrounding said electrode; annular metallic means through which said tubular insulating member and said electrode extend, and which annular metallic means is in direct metal to metal contact with the metallic shell or casing of the plug; and insulating members between said head and the innerend of said annular metallic means, and between the outer end of said metallic means and said terminal connector, and which insulating members surround the outer and inner ends of said tubular insulating member; said annular metallic means having an equalizing passage whereby pressure inside an engine cylinder, and pressure within interstices between said central electrode and said metallic means, are equalized when the plug is in use; said annular metallic means, said tubular insulating member and said insulating members collectively forming a support whereby said central electrode is supported within the metallic shell or casing of a spark plug.

3. In' a spark plug, a central electrode having a terminal connector at its outer end, and the inner end of which terminates adjacent the shell electrode of the plug; annular metallic means through which said central electrode extends, and which annular 'metallic means is in direct metalto metal contact with the metallic shell or casing of the plug; and insulating means whereby said central electrode is insulated from said metallic means, and which metallic and insulating means together provide a support for said central electrode which is removable as a unit from, and replaceable as a unit within the metallic shell or casing of the plug: said annular metallic means having an equalizing passage whereby pressure inside an engine cylinder, and pressure within interstices between said central electrode and said supporting means thereior are equalized when the plug is in use.

HECTOR RABEZZANA. 

